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  • Author Author: jw0752
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Reverse Engineering a GFI Plug

jw0752

While getting dinner tables ready today for the coming Christmas Dinner the boss (Millie) asked me to get some wrinkles out of a plastic table cloth. After retrieving my shop hair dryer I began to heat the plastic cloth to remove the wrinkles. After I had finished I noticed the little GFI Plug, on the end of the cord, plugged into the wall outlet. I thought to myself, you know I have never tested the GFI,and so I pushed the test button. The GFI however did not click and open but rather began to buzz that nasty 60 cycle buzz that means something is wrong and before I could pull the plug from the wall it let loose with a plume of smoke.

 

Opportunity had knocked as I have never investigated a plug based GFI before. Several minutes later I had it disassembled and off the cord. Since I was doing it anyway I decided to document the exploration so you too could see what was inside.

 

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After removing the covers I found the inner mechanism to be a nice mix of the electronic and the mechanical.

 

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The manual red Reset button engages a mechanism that holds the two mains switches closed and one smaller switch that leads to the Test Circuit. A small solenoid (Yellow Center) latches the mechanism in place. If the solenoid is energized the latch is pulled out of the way allowing the mechanism to drop and the three switches go open. A quick inspection and measurement with the ohmmeter showed that the solenoid coil had been the source of the smoke as it was now open. I removed the main transformer which had 2 common mode windings (the Large Red and Large Blue), one Test Winding ( Small Blue) for manually triggering the solenoid, and one Sense Winding (Small White) that led back to the electronics under the transformer.

 

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This opened the board itself for better analysis.

 

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The board was a pleasure to reverse engineer as all the components were well marked for identification and the single sided board with 14 components gave up its secrets quickly. Here is a schematic of the GFI device. If you click on the schematic it will open a larger view.

 

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I found it fascinating that the designer put the coil of the solenoid in series with the line side of the bridge rectifier. The rectified side of the bridge was connected directly to the cathode and anode of SCR EC103.  If the conditions in the circuit are such that the SCR is triggered it creates a short that applies sufficient current to the solenoid coil to pull the latch and open switches SW1, 2, and 3. The conditions for triggering the solenoid are #1, the Test button is pushed and sufficient current is induced in the Sense Winding of T1 by the Trip Winding. or #2 an imbalance in the current through Common Windings of T1 caused by Mains current leakage to ground rather than returning through Neutral also induces current in the Sense Winding.

 

My further analysis of the electronics is more guess than knowledge. Diode D2 should remove one side of the AC Current produced by the Sense Winding and the 5.6 Volt zener ZD1 probably adds a DC bias to the signal to put the gate of the SCR closer to its trigger point. The data sheet on the EC103 says that it is a sensitive gate SCR which makes sense as we would not want any more delay than necessary before energizing the solenoid. I hope someone corrects and improves on this analysis if they have time so I can understand it better. I have linked the data sheet in case it is helpful in better understanding the circuit.

 

http://datasheet.octopart.com/EC103M-Teccor-datasheet-5013.pdf

 

The hair dryer which is a shop tool no longer has a GFI plug and has been labeled for safety concerns. Unfortunately the way this GFI failed would not have protected the user in the case of a real emergency. When the solenoid coil burned out without tripping the mechanism it left the mains switches closed and the current still flowing to the hapless victim.

 

Thanks John

 

 

Parents

  • Hi John,

     

    This is very interesting - glad that all is well and you were around to unplug and examine the item. I too have RCDs for some devices, and so after seeing your analysis and reverse engineering, and comment, it is now clear that one should be prepared for that condition when the test button is pressed. Ordinarily I would not have given it a seconds thought, I'm pretty sure the user instructions do not advise in such detail.

     

    I tried to examine the plug-in RCDs here briefly just now - they have security screws so I didn't open them (I don't have a spare one).

  • in reply to shabaz

    This is what's inside a UNI-COM one.

     

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    This is the back of the board - the solenoid is inside the housing to the left and the core with its windings is to the right.

     

    image

     

    Obviously a broadly similar principle to John's, but there's a small IC on the front so the current trip point is possibly a bit more accurate.

     

    Even functioning properly I wouldn't trust it to save my life. It's worth having there, but at 30mA the trip point is too high to be certain of saving you anyway.

  • in reply to jc2048

    Hi Jon,

     

    Thanks for popping a different unit open. It is always interesting to see how different designers approach the same problem.

     

    John

  • in reply to jc2048

    Even functioning properly I wouldn't trust it to save my life.

    Here in NZ and Australia, they are mandatory on all new builds, and certain modifications to houses.

    They are fitted at the switchboard, and while they do have a test function, you can bet most people don't test them regularily.

     

    It's worth having there, but at 30mA the trip point is too high to be certain of saving you anyway.

    The tripping of them is 300/500mS, so while 30mA is the lowest trigger point, they have determined that the false tripping v safety means it's a reasonably safe comprimise.

     

    You can get 10mA versions and these need to be fitted under certain conditions. One of them is medical and the other is childcare facilities.

    Electricity (Safety) Regulations 2010 (SR 2010/36) (as at 04 April 2016) 24 Electrically unsafe RCDs – New Zealand Legis…

     

     

    I've never actually pulled one apart, but during our EST the basics of them was shown.

    They simply detect an inbalance between phase (or phases) and neutral current, which by deduction means the current is flowing to earth (which for us down under is tied to neutral)

    You can purchase proper testing devices ( would be a good RT) that allow you to set the current, but the test for consumers is simply a button which has a small load which bypasses one sense winding.

     

     

    The switchboard versions stay ON until tripped.

    Portable devices require resetting whenever the power is applied, so in theory you would be better using a portable device as at least you know the mechanical aspect trips.

     

    Cheers

    Mark

  • in reply to mcb1

    I think it is a matter of perception of safety and in a lot of cases, you could survive a short burst of current if you are healthy.

     

    As with any safety device, you have to maintain them and test them to ensure that they still work.

     

    Make no mistake, all devices fail, it is just a matter of time.

     

    I was surprised that there was not more self monitoring capability built into the device.  I guess making them cheap is more important than making them more robust.

     

    Just my perception.

     

     

     

    DAB

  • in reply to DAB

    As with any safety device, you have to maintain them and test them to ensure that they still work.

    or design them to be "fail to safe".

    The portable RCD's we have are fail to safe, as they trip when there is no power, meaning they need to be held ON, rather than energised off.

     

     

    Sadly this 'fail to safety' mentality is not well understood by a lot of our Engineers.

    They understand about presenting a closed contact so that any break in the wiring is detected, but sadly miss that they have to energise the relay to break the contacts.

    It's almost like they are scared to continuously power a relay.

     

     

     

    I'm a little shocked that the design allowed the device to effectively cook the solenoid coil.

    If there had been a Ground Current, then it wouldn't have tripped (possibly electrocuting the user) and then possibly set fire to something.

     

     

    Mark

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  • in reply to DAB

    As with any safety device, you have to maintain them and test them to ensure that they still work.

    or design them to be "fail to safe".

    The portable RCD's we have are fail to safe, as they trip when there is no power, meaning they need to be held ON, rather than energised off.

     

     

    Sadly this 'fail to safety' mentality is not well understood by a lot of our Engineers.

    They understand about presenting a closed contact so that any break in the wiring is detected, but sadly miss that they have to energise the relay to break the contacts.

    It's almost like they are scared to continuously power a relay.

     

     

     

    I'm a little shocked that the design allowed the device to effectively cook the solenoid coil.

    If there had been a Ground Current, then it wouldn't have tripped (possibly electrocuting the user) and then possibly set fire to something.

     

     

    Mark

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